The invention relates to thermochemical cycles for hydrogen production and more particularly to sulfur trioxide (SO.sub.3) decomposition reactors utilized in thermochemical cycles for hydrogen production.
Hydrogen, a valuable raw material for the petroleum and petrochemical industries, is expected to become by early in the next century an important renewable-based, transportable fuel either by itself or in some hydrocarbon form such as methanol. Hydrogen can be produced through the decomposition of water by means of thermochemical cycles which reduce the high temperature requirements of the 3000 K. (degrees Kelvin) straight thermal decomposition process to the 1200 K. levels that can be generated in nuclear fission or fusion reactors or in high intensity, focused solar reflectors.
An example of a thermochemical process for producing hydrogen is the sulfur-iodine cycle being developed by the General Atomic Company. The essential steps of the sulfur iodine cycle are represented by the following reactions: EQU 2H.sub.2 O+SO.sub.2 +xI.sub.2 .fwdarw.H.sub.2 SO.sub.4 +2HI.sub.x ( 370-390 K.) EQU 2HI.sub.x .fwdarw.H.sub.2 +xI.sub.2 ( 393 K.) EQU H.sub.2 SO.sub.4 .fwdarw.H.sub.2 O+SO.sub.2 +1/2O.sub.2 ( 1144 K.)
The dominant energy requirements, heat versus temperature, are necessary in this process for the H.sub.2 SO.sub.4 concentration and vaporization, conversion of H.sub.2 SO.sub.4 into SO.sub.3 +H.sub.2 O, and SO.sub.3 decomposition steps.
The SO.sub.3 decomposer is the critical process unit in nearly all the viable thermochemical plants to produce hydrogen. These plants can be driven by high temperature gas reactors, solar collectors or fusion reactors, utilizing sodium, potassium or helium as heat transfer fluids to supply the large heat demand of the SO.sub.3 decomposer. Catalysts are required in the decomposer in order to keep the temperature required to reasonable levels of 1070-1120 K. The key requirement is to supply heat to the catalytic surfaces where the endothermic SO.sub.3 reaction occurs. This SO.sub.3 decomposition produces SO.sub.2 and O.sub.2 for the thermochemical production of hydrogen.
Measured SO.sub.3 kinetics and equilibrium show this high temperature SO.sub.3 decomposition reactor to be surface kinetics (heterogeneous) controlled at lower temperatures, below 1050 K., and homogeneous at higher temperatures, above 1180 K. For non-catalytic surfaces the conversion from SO.sub.3 and SO.sub.2 is about 20-30% over the temperature range 1080 K. to 1180 K. for a 0.3 to 1 second residence time at around 1.5 atm. total pressure. The low conversion leads to large recycle H.sub.2 SO.sub.4 flows and thus much larger and more expensive equipment. Increased residence time improves the kinetics but increases the size of the equipment. Increased total pressure decreases the equipment size but unfavorably shifts the equilibrium, and decreased conversion increases equipment size. Catalytically enhanced kinetics greatly improve the conversion to the range of 65-80%. It is desirable to operate at a temperature of around 1050 K. in order to eliminate the need of very expensive platinum catalysts and allow substitution of much less expensive CuO or Fe.sub.2 O.sub.3 catalysts.
The design of a chemical reactor with fast kinetics and large associated heat effects is very difficult. A design of least cost and greatest simplicity is desired. Catalytic decomposers heated by internal heat exchangers appear to be too large to be cost competitive with other hydrogen production technologies. The most obvious choice, a packed bed reactor, does not appear feasible because heat transfer from in-bed heat exchangers to the packed bed of catalysts is very inefficient and requires extremely large temperature gradients between the heat exchanger fluid and the packed bed. Costly, high heat transfer media flow rates are also required, and large radial temperature gradients appear within the bed between the internal heat exchanger tube elements. Fluidization of the bed of catalysts greatly reduces the temperature differences between the heat transfer fluid and the catalyst surface. However, substantial pumping power is required to fluidize the bed, resulting in a higher operational cost design.
Fusion reactors offer some unique advantages as drivers for thermochemical hydrogen plants. Thermal heat from the blanket of a tandem mirror fusion reactor can be utilized. One particular tandem mirror blanket concept is a lithium-sodium, liquid metal 50% weight mixture in the cauldron blanket module. Helium or sodium can be used as the heat transfer fluid to carry heat outside the nuclear island to process exchangers within the thermochemical hydrogen production cycle. Either a direct condensing vapor heat exchange loop or a heat pipe driven loop can be utilized. Problems with this design, however, include the safety problems of the isolation of liquid metals from the process stream and the permeation of radioactive tritium into the product stream.
Thermochemical cycles, the interface with thermal reactors, fluidized bed decomposer designs, and associated problems, are described in UCRL-84212, "Interfacing the Tandem Mirror Reactor to the Sulfur Iodine Process for Hydrogen Production", T. R. Galloway, Lawrence Livermore National Laboratory, June 1980, and UCRL-84285, "The Process Aspects of Hydrogen Production Using the Tandem Mirror Reactor", T. R. Galloway, Lawrence Livermore National Laboratory, September 1980, which are herein incorporated by reference.
It is accordingly an object of the invention to provide a low cost and high efficiency SO.sub.3 decomposer for a thermochemical hydrogen production process.
It is also an object of the invention to provide a catalytic SO.sub.3 decomposer which can be interfaced with a tandem mirror fusion reactor at 1200 K. or below.
It is another object of the invention to provide a SO.sub.3 decomposer with improved safety barriers and added modularity for increased reliability.
It is also an object of the invention to provide a SO.sub.3 decomposer which can interface with a fusion reactor which has improved tritium processing and isolation features.
It is another object of the invention to provide a SO.sub.3 decomposer which operates at a temperature to allow the use of inexpensive CuO or Fe.sub.2 O.sub.3 catalysts in place of more expensive platinum catalysts.
It is yet another object of the invention to provide a SO.sub.3 decomposer which provides a high efficiency transfer of heat from the source to the catalyst.
It is still another object of the invention to provide a SO.sub.3 decomposer which has a high conversion efficiency.